The present invention relates to manway cover tensioning systems and in particular to a means for handling closure members.
Manway covers as referred to are used to close access apertures in pressure vessels such as steam power generators in nuclear power plants. Very high compressive forces are needed to seal such openings. Hundreds of thousands of pounds of compressive force may be applied. A plurality of tensioning studs may each be tensioned to 50,000 to 100,000 pounds. Tensioning studs for large diameter closures has always been a problem. In the past, torque wrenches and bars have been employed to turn nuts with the intent of applying desired axial load on the studs. However, the axial force which is applied is a function of many factors in addition to the tangential force to be supplied by a wrench. They include coefficient of friction of the surface against which a nut is bearing. Another is friction between a thread and a stud itself. According to John H. Bickford in "The Initial Preload-What happened To It?", Mechanical Engineering, October 1983, there are some 76 variables that affect the relationship between the torque applied to a nut on a tensioning stud and the actual compressive force applied thereby due to the tension in which the stud has been placed. Further, in addition to being unreliable in the terms of precisely applying tensile force to a stud, torqueing can cause uneven stud tensioning, flange distortion, leakage, stud damage and injury to workers.
Therefore, systems have been developed in which studs which will hold a manway cover are stretched, such as hydraulically, to a desired tensile stress. Closure nuts are threaded to retain the manway cover. The force stretching the studs is then removed, and the studs are tensioned to compress the nuts against the manway cover. The closure member handling system must handle both a manway cover and tensioner ring in cooperation with each other.
Tensioner rings can be heavy and weigh on the order of hundreds of pounds. They are difficult to maneuver and must be held in place while the bolting process is performed. Prior art systems for handling tensioning rings include floor track-mounted systems in which a tensioning ring is supported to a support means slidable in a track to bring the tensioning ring into or out of engagement with the manway. Such systems require time-consuming realignments at each use due to thermal expansion due to the high temperature environment in which they commonly operate. Difficulties in use can also result in errors in alignment. Systems are generally limited in the degree of freedom of movement afforded in a tensioning ring as well.
In a floor-mounted track type system a closure member must be supported in a cantilivered manner, thus maximizing the stress applied to support components. In the case of nuclear power plants, the new installation of dedicated, installed floor systems provides a further problem of a structural modification. Structural changes require seismic requalification of the stucture.